The present invention relates to the field of metallurgy and in particular to the melting of metals and alloys in plasma melting furnaces in which plasma burners of high output are used.
The plasma melting torches of high power used up to now for the melting or remelting of metallic materials use technically pure argon as the operating gas. This operating gas, on the one hand, protects the highly heated tungsten cathode within the burner from burning up and determines essentially the composition of the furnace atmosphere over the molten material and thus the basic electric parameters of the plasma column, such as voltage gradient along the column, arc voltage and arc temperature of the plasma column. From this there arose the idea of controlling these electric arc parameters by the admixture of diatomic gases, for instance to increase the output of the burner with constant current intensity by increased arc voltages and to influence the course of the melting through the utilization of chemical reactions between the molten material and a furnace atmosphere which is suitably adjusted by means of the operating mixture. However, one prerequisite for operation with gaseous mixtures was that the hot tungsten cathode not come into contact with oxidizing gases, so as to avoid the burning up of the cathode which would then take place. Oxidizing gas mixtures therefore drop out of consideration from the very start as operating gas for such melting burner designs. The use of other cathode materials which remain operable in an O.sub.2 -containing operating gas without being burned up, such as also used for instance in plasma cutting torches, for example zirconium oxide cathodes, was possible heretofore only with low current intensities. An increase of the output of plasma burners for the melting of metallic materials was not possible with the known solutions.